Add missing standard library includes for self-sufficient headers

Fixes builds with newer GCC/libstdc++ that no longer provide <cstdint>,
<cstring>, <cfloat>, <memory>, <algorithm> etc. transitively. Also
disambiguates visit<> calls in taxonomy.h with the full namespace and
casts the character value in IfcCharacterDecoder to uint32_t to silence
ambiguous overload warnings.

src/ifcparse/IfcBaseClass.h

@@ -27,6 +27,7 @@
 #include "utils.h"
 
 #include <atomic>
+#include <cstdint>
 #include <boost/shared_ptr.hpp>
 
 class aggregate_of_instance;